In recent years, total station devices have gained popularity for both surveying and machine control applications. A total station is an optical instrument that is capable of sighting a target and determining the azimuth and ordinate angles from the total station to the target, and also the distance from the total station to the target. The distance is determined by directing a beam of laser light at the target, sensing the reflected light returning from the target, and determining the time of flight of the beam. Since the relative orientation and distance of the target from the total station are then known, and since the precise location of the total station will have already been determined with conventional surveying techniques, the location of the target is therefore determined.
Robotic total stations have been developed that track a target continuously, without the need for an operator. Servo motors in the robotic total station cause it to rotate toward the target, providing precise angular and distance measurements. Typically, the target will be carried by a surveyor or by an earthmoving machine, driven by an operator. The robotic total station continuously transmits the data defining the position of the target to a computer system that may be carried by the surveyor, located on the earthmoving machine, or located at a remote location, thus providing real time position data for the target. Robotic total stations may provide position information for machine control, communicating the information for comparison to the job plan.
The robotic total station tracks and measures the position of the moving target remotely, continuously sending the measured data to the controlling computer. Robotic operation of a total station can be combined with real time kinematic measurements made by GPS receiver system to enhance the overall accuracy and versatility of the system. Such a system contemplates a GPS antenna being carried with the total station target so that position data can be collected through both the total station system and the GPS system. Other information may also be provided and combined to determine the position of the target or antenna. For example, location information, such as that developed by other sensors, such as laser scanners, echo beams, or atmospheric condition sensors, may also be used to enhance the accuracy of the system.
While such systems are advantageous, a need exists for a system which is capable of tracking one or more targets at a construction site where there may be multiple targets on various pieces of equipment at the site, and which permits GPS position data and total station position data to be combined readily. It is typical to provide a GPS guidance system for construction equipment. Although it is very accurate in operation, such a GPS guidance system does not operate well, or ceases to operate at all, when one or more of the GPS satellites upon which it bases location calculations becomes obscured from the view of the GPS satellite antenna. There are situations at a construction or mining site when there is either poor GPS coverage or GPS shading occurs, for example as a result of working adjacent to high walls or working under a bridge or overpass. If this should occur, the construction equipment can be left without needed position data for an extended period, significantly reducing productivity as the operator of the machine is forced to operate using alternative position determination equipment, such as an inertial reference unit. Clearly this is undesirable due to the added equipment cost.